Ubuntu LEB are images built out of Ubuntu Launchpad. An image is a 'collection' of ubuntu packages (packages as in .deb). each binary package is built on Launchpad builders, natively on ARM devices. when a developer is *done* with something he would build a source package locally (.dsc + sources) and *push* it into Launchad (either into the main archives if application, or most likely into the Linaro 'overlay' PPA : https://launchpad.net/~linaro-maintainers/+archive/overlay/). Then a builder will build the package locally (you can check the build log on the PPA). typically a source package build on a builder is made of:
1- boot a minimal rootfs (if you build for 12.04, it uses a 12.04 root fs) 2- run apt-get build-dep <the package to build> 3- run dpkg-buildpackage -b 4- archive the generated artifacts (.deb, logs, ..) into PPA
then regularly packages are assembled into images.